Fuel cells are devices that provides for clean and relative efficient conversion of matter into electrical energy and heat. A range of different technologies have been developed within the last couple of decades, each employing its own principle, type of reactants, optimum operation conditions etc. One technology which has gained particular interest in recent years is the so-called PEM fuel cell.
A PEM (proton exchange membrane) fuel cell comprises an anode and a cathode and a proton exchange membrane interposed there between. The proton exchange membrane comprises a catalyst on the side facing the anode as well as on the side facing the cathode. The principle of a PEM fuel cell is that supplying hydrogen to the side of the membrane facing the anode by virtue of the catalyst on the side of the membrane facing the anode results in the chemical reaction:Anode reaction: H2−>2H++2e−  (1)
The anode is made of an electrically conducting material and thus transports the electrons generated on the anode side of the membrane, whereas the protons generated on the anode side of the PEM-membrane diffused through the membrane.
On the cathode side of the membrane oxygen (or air) is supplied. If an electrical load is connected between the cathode and the anode of the cell so as to form an electrical circuit, the electrons generated at the anode flows through this load to the cathode. The oxygen supplied to the cathode side of the membrane by virtue of the catalyst on the side of the membrane facing the cathode reacts with the protons which have diffused through the membrane and the electrons flowing to the cathode according to the following chemical equation:Cathode reaction: O2+4H++4e−−>2H2O+heat  (2)
Hence, the net reaction taking place in a PEM fuel cell is:2H2+O2−>2H2O +electrical power+heat  (3)
One PEM fuel cell is capable of generating a voltage of 1.23 V. In order to achieve higher voltages for PEM fuel cells, a number of PEM fuel cells are usually connected in series in a so-called PEM fuel cell stack. A fuel cell stack is for the sake of economy often designed in a way that integrates the cathode of one fuel cell with the anode of an adjacent fuel cell of the corresponding stack. This is achieved by employing so-called bipolar plates. A bipolar plate is a plate which has two sides, one of which functions as an anode for one fuel cell, and the other of which functions as a cathode for the adjacent fuel cell in the corresponding fuel cell stack.
A PEM fuel cell is quite sensitive in the sense that it is important that the hydrogen supplied to the anode is not contaminated with other gases. Furthermore, due the heat generated on the cathode side, it is important that this side is cooled in order for the fuel cell stack to remain within the temperature limits which provide for optimum performance. A fuel cell stack may be cooled by supplying to the cathode side of each cell more oxygen (or air) than is needed relative to the amount of hydrogen supplied on the anode side of each cell. Another principle of cooling is the implementation of a coolant circuit in the stack.
On the basis of the above considerations much research and development of PEM fuel cells has in the recent years focused on the specific physical design of the fuel cell and in particularly on the physical design of the bipolar plates.
US patent application No. 2003/0059664 discloses bipolar plates in a stack formation, where the bipolar plate is sandwiched between an oxygen and a hydrogen electrode adjacent to the electrolyte chambers separated by the bipolar plate. The electrodes are formed with channels in a grid like fashion, making the formation production-wise complicated and expensive.
A more simple stack formation is disclosed in International patent application WO 2007/003751 as a prior art example, where the bipolar plates are sealed against the electrode by elastomeric sealants formed as square o-rings. These o-rings do not allow oxygen or hydrogen passing by, such that a different, improved system is proposed, which however has a production-wise complicated configuration.
WO 03/077341 discloses a bipolar stack formation with bipolar plates having channels running from edge to edge of the bipolar plate. The channels are sealed against the electrolyte by a plate of fibrous composite material. In order to achieve a gas tight arrangement, the dimensional tolerances for the fibrous plate are narrow, which production-wise is complicated.
Accordingly numerous different designs of bipolar plates have been disclosed in the art. However, although a substantial amount of these designs fulfil the technical requirements for such plates, they all suffer from the disadvantage that the have a structure which is quite complicated and therefore their manufacture is costly. This is in particular the case in situations wherein in addition to the dipolar plates, also cooling plates are included in the fuel cell stack.